Field of the Invention
One disclosed aspect of the embodiments relates to an electrochromic element controlling the intensity and color of light, a lens unit including the electrochromic element, an imaging device including the electrochromic element, and a window member including the electrochromic element.
Description of the Related Art
Electrochromic elements are those including a pair of electrodes and an electrochromic layer placed between the electrodes. The amount of light passing through the electrodes can be regulated in such a manner that the transmittance of a compound in the electrochromic layer is varied by applying a voltage to the electrochromic layer from the electrodes.
In recent years, in videographic systems including a solid-state imaging element, there have been increasing demands for variable neutral density (ND) filters capable of steplessly regulating optical density. Many optical elements containing a liquid crystal or including an inorganic electrochromic thin-film have been proposed for this application. However, the optical elements are inferior in light regulation range or reliability to known ND filters and therefore have not been widely used.
Meanwhile, optical elements containing organic electrochromic molecules have a wide light regulation range, are easy in designing spectral transmittance, and therefore are particularly promising for applications such as variable ND filters mounted in imaging devices.
An electrochromic element includes an electrochemically active anodic material and electrochemically active cathodic material placed between a pair of electrodes. At least one of the electrochemically active anodic and cathodic materials is an electrochromic material, that is, a material that exhibits an absorption band in the visible light region by electrochemical oxidation or reduction. The oxidation of the anodic material and the reduction of the cathodic material occur simultaneously on the electrodes. This forms a closed circuit in the electrochromic element to allow a current to flow.
When the electrochromic element has a large area, the influence of a voltage drop in a surface of each electrode is particularly large. This is because the resistivity of a material contained in the electrode is two orders of magnitude greater than that of metal.
International publication No. WO 2011/085917 (hereinafter referred to as Patent Document 1) discloses a light-emitting element in which the luminance in a surface of an electrode is made uniform in such a manner that a voltage drop gradient is compensated for by monotonically varying the conductivity of the electrode in proportion to the distance from an electrical terminal.
Japanese Patent Laid-Open No. 2005-321521 (hereinafter referred to as Patent Document 2) discloses an electrochromic element in which the influence of a voltage drop is reduced by regulating the ratio of the resistance of an electrode to the resistance of a solution such that non-uniformity during coloring is improved.
The light-emitting element disclosed in Patent Document 1 is one that reduces the voltage drop in a surface of an electrode to achieve uniform light emission. Patent Document 1 does not describe how to reduce the transmittance non-uniformity of light passing through the electrode in the electrode surface. Therefore, the light-emitting element cannot reduce transmittance non-uniformity.
Patent Document 2 describes that the coloring uniformity of the electrochromic element is increased by regulating the relationship between the resistance of a pair of electrodes, the length of the electrodes, the electrical conductivity of an electrolytic solution, and the distance between the electrodes. However, regulating the relationship therebetween reduces the degree of freedom in configuration of the electrochromic element.